Update: What are your chances of getting HIV?

March 1, 2002

What are your chances of getting HIV?

— Cecil Adams

Tolja

Veteran readers of this column will recall Cecil's observation years ago that the best way to reduce your risk of HIV infection — I said AIDS at the time, because back then everybody who got HIV got AIDS and there didn't seem much point in drawing a distinction — wasn't using condoms, the standard advice at the time. Rather, it was being fussy in your choice of sexual partners. Now comes a new study from researchers at the Centers for Disease Control ("Reducing the Risk of Sexual HIV Transmission," Varghese et al, Sexually Transmitted Diseases, January 2002). And you know what? I was right.

Granted, with the advent of new drug therapies, fear of HIV has subsided among both gays and straights. Granted also, this new study involves only slightly less guesswork than the one I based my original conclusion on. Still, to the extent that your higher cortical functions are engaged when you're about to have sex, understanding how different behaviors affect your chance of getting HIV may reduce your risk of getting other sexually transmitted diseases.

Let's start with some numbers. The stats in the table indicate the relative risk of various sex partners and sex acts. Within each category, the first item is the baseline, the second item is x times riskier, and so on. For example, our experts have decided that for purposes of comparison "insertive fellatio" — getting a blow job — is the least risky two-person sex act. Being the bottom during anal sex is 100 times riskier.

To figure the combined risk of multiple behaviors, you multiply the relative risk numbers. In the table the baseline behavior (least risky by definition) is wearing a condom while getting fellated by someone who's tested negative for HIV. The riskiest behavior is being the bottom during unprotected anal sex with someone who's HIV positive. For gays this is 860,000 times more dangerous than their baseline, and for heterosexuals the risk increases by a factor of 9,412,000. That good old-fashioned blow job is looking better all the time.

To get back to my original point, the most obvious lesson of these numbers is to be choosy about whom you have sex with. For gays, having sex with someone who has HIV is 430 times more dangerous than with someone who's tested negative for HIV; for straights, who have a lower rate of HIV to start with, the risk factor is 4,706.

But the type of sex act is still important, particularly if you don't know your partner's HIV status. (Admittedly it's not an easy subject to bring up early in the game.) Being the recipient in anal sex is five times riskier than being the recipient in vaginal intercourse and 50 times riskier than performing fellatio. By the way, if you're wondering why cunnilingus isn't listed, the researchers say there's virtually no risk data for it. Not that I'd encourage the Teeming Millions to court danger, but this sounds like a golden opportunity to do one's duty for science.

One might quarrel with many of the numbers above. For example, the idea that condom use decreases your risk by a factor of 20 strikes me as optimistic — a factor of 10 (at best) is more like it. I'd also be foolish to claim that risk of other STDs can be extrapolated from HIV risk.

Still, any information has to help. We're long past the point where the only danger in sex is that your partner won't respect you in the morning.